In the worldwide consciousness about energy conservation, reduction of power consumption in such products as home-use refrigerator freezers and the like appliances is urged. Many of the compressors in these appliances are inverter-controlled and driven at lower operation frequencies. However, improvement in the stability of compressor performance during low speed operation still remains a task to be solved, and improvement in the efficiency is another task.
Conventional compressor technology is described using a compressor disclosed in Japanese Patent Unexamined Publication No. 2000-145637, etc. as the example. The up-down disposition of a compressor's constituent elements is described based on a typical configuration among the conventional compressors.
FIG. 13 shows a vertical cross sectional view of a conventional compressor, FIG. 14 shows a horizontal cross sectional view, and FIG. 15 shows a perspective view of a conventional piston as seen from the above.
As shown in FIG. 13, sealed housing 1 contains refrigerant 15 which is filling the inner space of the housing, oil 2 which is stored at the bottom, motor element 5 consisting of stator 3 and rotor 4 having a built-in permanent magnet, and compression element 6 which is driven by motor element 5.
Compression element 6 is described below.
Crankshaft 9, which is disposed vertically, includes main shaft 7 and eccentric shaft 8. Crankshaft 9 has built-in oil pump 20, which pump is connected through to the top of eccentric shaft 8 via spiral groove 17. An open-end of oil pump 20 at the bottom is dipped in oil 2. Cylinder block 12 supports main shaft 7 so that the shaft can make a free revolution, and has cylinder bore 11 for forming compression chamber 10.
Piston 50 is inserted in cylinder bore 11 for reciprocation. Piston pin 14 of a cylindrical shape is disposed in parallel with eccentric shaft 8, and pin 14 is held in piston-pin hole 51 provided in the piston. Connection structure 13 has major connection hole 33 for insertion of eccentric shaft 8, minor connection hole 31 for insertion of piston pin 14, and rod 32 which couples eccentric shaft 8 with piston 50 via piston pin 14.
FIG. 15 illustrates piston 50 with the end for coupling to crankshaft 9 at this side of a viewer, as seen from above the compressor. Piston 50 has an approximate cylindrical shape, which is symmetrical in terms of the right and left sides. As to the both ends of the piston, the surface which constitutes compression chamber 10, in collaboration with cylinder bore 11, is called piston top surface 52, whereas the other end surface connected with connection structure 13 is called piston skirt surface 53. In FIG. 15, piston skirt surface 53, is at the bottom side of the drawing.
The above-configured compressor operates in the following manner.
When motor element 5 is driven with electric power, rotor 4 starts rotating clockwise (as viewed from above the compressor), causing crankshaft 9 to also rotate. The rotating motion of eccentric shaft 8 is conveyed to piston 50 via connection structure 13 and piston pin 14. Then, connection structure 13 oscillates with respect to piston pin 14, and piston 50 reciprocates within cylinder bore 11. As a result of reciprocating motion of piston 50, refrigerant 15 which is filled in sealed housing 1 is sucked into the inside of compression chamber 10, and is compressed and then discharged to the outside of sealed housing 1. This cycle is repeated.
When crankshaft 9 starts rotating, oil pump 20 starts sucking oil 2 and the oil is brought upward through spiral groove 17. The oil is jet-scattered from the top end of eccentric shaft 8 to lubricate such sliding surfaces as a surface between minor connection hole 31 of connection structure 13 and piston pin 14 and a surface between piston 50 and cylinder bore 11.
The above-described conventional hermetic compressors, however, sometimes exhibit unsymmetrical wear at a surface of sliding-contact between piston 50 and cylinder bore 11, which are the constituent parts of compression element 6, when used in the refrigeration system of home-use refrigerator freezers which may be operated at a low revolution speed (for example, at an operation frequency of 1500 r/min).
The inventor of the present invention tested a conventional hermetic compressor driven at low operation speed to observe the posture of piston 50 in cylinder bore 11. It was found that the surface of sliding-contact had unsymmetrical wear. The wear began from a point in the right portion of piston skirt surface 53, as viewed from above the compressor with crankshaft 9 at this side of a viewer, with respect to a vertical plane containing the center axis of piston 50 (viz. point L in FIG. 15), and a point in the left portion of piston top surface 52 (viz. point H in FIG. 15). Namely, piston 50 in a tilt posture was colliding against cylinder bore 11.
When the wear due to contact develops, a gap is generated between piston 50 and cylinder bore 11, which leads to a leakage of refrigerant 15 during the sucking and compression cycle. This invites instability and/or deterioration in the performance of a compressor, making it difficult to guarantee the operational reliability for a long-time.
On the other hand, when an anti-wearing measure was tried with piston 50 and cylinder bore 11 by means of the mechanical design, material used, etc., the complexity in the structure waa increased, thereby increasing manufacturing cost and the like.